A fossil-fired steam generator generates superheated steam using the heat that is generated by combustion of fossil fuels. Fossil-fired steam generators are mainly used in steam power plants, which are primarily used to generate electricity. The generated steam is supplied to a steam turbine in this case.
In a similar way to the various pressure stages of a steam turbine, the fossil-fired steam generator also comprises a plurality of pressure stages featuring different thermal states of the water-steam mixture which is contained in each case. In the first (high) pressure stage, the flow medium on its flow path is carried firstly through economizers, which use the residual heat to preheat the flow medium, and then through various stages of evaporator and superheater heating surfaces. In the evaporator, the flow medium is evaporated, after which any residual moisture is separated off in a separating device and the remaining steam is heated up further in the superheater. The superheated steam then flows into the high-pressure part of the steam turbine, where it is expanded and supplied to the subsequent pressure stage of the steam generator. There it is superheated again (intermediate superheater) and supplied to the next pressure part of the steam turbine.
Due to all manner of external influences, the heat output transferred to the superheater can fluctuate significantly. It is therefore often necessary to regulate the superheating temperature. This is usually achieved by means of an injection of feed-water for the purpose of cooling before or after individual superheater heating surfaces, i.e. an overflow line branches off from the main flow of the flow medium and leads to injection coolers disposed there accordingly. The injection is usually regulated by means of fixtures in this case, using a reference value that is characteristic of the temperature deviations from a predetermined desired temperature value at the outlet of the superheater.
Modern power plants are expected to deliver not only high levels of efficiency, but also maximal flexibility of operation. In addition to short start-up times and rapid load changes, this also includes the ability to compensate for frequency disruptions in the power grid. In order to meet these requirements, the power plant must be capable of supplying power increases of e.g. 5% and more relative to full power within a few seconds.
Such changes in power within a period of seconds from a power plant block can only be achieved by means of coordinated interaction between steam generator and steam turbine. The contribution which the fossil-fired steam generator can make to this consists in the use of its stores, i.e. the steam store and the fuel store, and in rapid changes of the actuating variables for feed-water, injection water, fuel and air.
This can be done by opening partially throttled turbine valves of the steam turbine or a so-called step valve, for example, thereby decreasing the steam pressure ahead of the steam turbine. Steam from the steam store of the upstream fossil-fired steam generator is therefore withdrawn and supplied to the steam turbine. This measure results in a power increase within a few seconds.
This additional power can be released in a relatively short time, making it possible at least in part to compensate for the delayed increase in power that is produced by the increase in furnace output. The power of the whole block is immediately boosted as a result of this measure, and can be continuously maintained or exceeded by increasing the furnace output thereafter, provided the installation was in the partial load range at the time the additional power reserves were demanded.
However, permanent throttling of the turbine valves in order to maintain a reserve always results in a loss of efficiency and therefore, in order to ensure cost-effective operation, the extent of throttling should be kept as low as is absolutely necessary. Furthermore, some design formats of fossil-fired steam generators, e.g. once-through flow boilers, may have a significantly smaller storage volume than e.g. natural circulation boilers. In the method described above, the difference in the size of the store has an influence on the response to power changes of the power plant block. Moreover, particularly in the upper load range, the design pressure in the overall steam generator must not be exceeded as a result of throttling, and therefore this measure can only be applied to a limited extent or even not at all in the upper load range.